The functional significance of specific prolactin binding sites on ependyma of the rat choroid plexus is being investigated with emphasis towards a possible role for prolactin in the regulation of electrolyte balance between blood and cerebrospinal fluid (CSF). A preliminary electron microscopic autoradiographic analysis of I125-prolactin movement relative to ependyma cytology indicates initial binding of prolactin to ependyma plasmalemma 3 minutes post-injection. By 20 minutes post-injection, silver grains reveal internalization and partial movement of prolactin to lysosomal bodies. These results suggest that ependyma may be the final target cells rather than a component of a blood to CSF transport system for prolactin. EM autoradiography is being continued with sacrifice times of 40 and 60 minutes after I125-prolactin injections. CSF electrolyte concentrations are being measured in control and hyperprolactinemic rats. Hyperprolactinemic animals were established through the subcutaneous transplantation of prolactin-secreting pituitary adenomas. The project is also designed to localize specific prolactin binding sites within the rat central nervous system (CNS) exclusive of those on ependyma of the choroid plexus. This will be accomplished through light and EM autoradiography in combination with the principles of competitive binding. Prolactin will be administered both vascularly and through cerebroventricular injections. Prolactin exhibits a short-loop feedback system to the CNS and it is anticipated that this work will aid in determining those regions of the CNS involved with the regulation of prolactin secretion from the anterior pituitary gland.